It is well known that salivary components can interact with microbes to influence their colonization of the oral cavity. One such interaction involves the abundant salivary enzyme, amylase, which binds specifically and with high affinity to commensal oral streptococci that are early colonizers of the saliva-coated tooth and numerous in supragingival dental plaque. Amylase-binding streptococci (ABS) colonize only hosts with detectable salivary amylase activity suggesting that amylase interactions modulate oral colonization by ABS and have an evolutionary basis. The ABS S. gordonii produces two amylase-binding proteins (ABPs) [AbpA (20-kDa) and AbpB (82-kDa) of Streptococcus gordonii. The binding of amylase to this species is dependent only on AbpA in S. gordonii (and likely its homologs in other species), and not on AbpB. In vitro studies found ABPs to play a role in bacterial adhesion and biofilm formation. Interestingly, AbpA defective S. gordonii mutants were able to colonize rat mouths as well as the parental strains, suggesting that additional bacterial factors are involved in colonization and survival in vivo. In light of these findings, we have considered potentially novel functions for these proteins. Preliminary studies suggest that amylase-binding to S. gordonii elicits differential gene expression in the bacterial cell. Thus, th Specific Aims of this proposal are to: 1: identify streptococcal components that interact with AbpA that may participate in a novel signaling pathway. 2: determine the three-dimensional structure of AbpA, identify amylase binding motifs and to address the evolutionary basis for the divergence of ABP gene sequence. 3: compare the global gene expression response of S. gordonii following binding of amylase to the response induced by the binding of whole- and ductal-saliva and other purified salivary components (such as mucins, sIgA, etc). Knowledge of saliva-mediated bacterial signaling pathways could have clinical application, for example by devising analog agents that serve as inhibitors or promoters of microbial colonization. Such agents may enable selective manipulation of bacterial colonization, and by extension, impact disease prevention or control. This knowledge could well extend beyond biofilm development in the oral cavity, with application to other microbial communities affecting systemic disease.